Bull. ent. Res. 67, 185-204 185 Published 1977 .. Migrations of Dysdercus spp. (Hemiptera : Pyrrhocoridae) related to movements OP the Inter-Tropical Convergence Zone in West Africa DOMINIQ~DUVIARD * Laboratoire d'EntotnoZogie Agricole, Centre Orstom d'Adiopodoumé, B.P. V51, A bidjan, Zvory Coast Abstract The possibilities of migrations in the West African species of Dysdercus are discussed and a hypothesis of long-range migrations asso- ciated \with the Inter-TTopical Convergence Zone and its wind systems is proposed. Catches of adult Dysdercus in four light-traps distributed from south to north of the Ivory Coast showed that the phenology of assumed migratory activity in D. voelkeri Schmidt differs with latitude and may be correlated with particular types of weather; stainer migrations taking place during the warm, wet and sunny part of the year. The whole life cycle of the insects as well as their flight activity occur under these climatic conditions, prevailing in a belt of 600-900 km width, situated immediately to the south of the Inter-Tropical Front. Colonisation of newly available habitats is thus only possible when climatic factors allow: (i), migratory flight activity and (ii), survival in the colonised area. A close examination of the timing of both migrations in the two main species, D. voelkeri and D. mehoderes Karsch, and of annual movements of the I.T.F. leads to the only logical hypothesis that the transportation of migrating insects is effected by atmospheric convergence, prevailing wind currents and air mass displacements. Introduction Migration of adults of a new generation from one breeding site to another is an important feature of the biology of many insect species (Southwood, 1960; Johnson, 1969; Bowden, 1973; Dingle, 1974) and migrants must therefore be considered as active colonisers of every potential habitat, and not just as individuals leaving an unsuitable environment (Dingle, 1972). This recent concept of migratory behaviour modifies drastically the concept of colonisation of new habitats, such as a field, for example, by pests. Since the early work of Ballard 8c Evans (1928), Golding (1928), Bebbington & Allan (1936), and others, reviewed by Pearson (1958), little field work has been done on migrations of cotton stainers (Dysdercus spp.). The accent in studies on migration has been on physiological aspects in the work of Edwards (1969n, 6, 1970), Dingle & Arora (1973) and Dingle (1972, 1974) and field work has been restricted, until recently, to occasional observations by light-trapping (Gibbs & Leston, 1970; Bowden, 1973) or to the classical type of cotton field survey (Pierrard, 1972). Curiously, the idea that stainers * Present address : Laboratoire d'Entomologie Agricole, Services.Scientifiques Centraux de - l'O.R.S.T.O.M., 70-74 Route d'Aulnay, 93140 BONDY, France. ~I (L 2283) A i- . .L 186 DOMINIQUE DUVIARD are common migrants is based on old observations which have not attracted much interest from field workers, possibly because these insects can be controlled easily with pesticides. In the laboratory, the life of adult stainers, at least of the females, may be separated into two distinct phases: (i), a migratory period of enhanced locomotory activity by young, non-feeding, sexually immature imagos with well-developed flight muscles; (ii), a subsequent period during which feeding and mating induce flight muscle histolysis in the females and in some males (which are then unable to fly), and also induce the maturation of ovaries in females. These two phases also exist in the field, where differences in behaviour have been observed (Duviard, 1972). During the migratory period, the adult stainers make nocturnal flights, which end with the discovery of the host plant, followed by the reproductive and sedentary period. Such migratory flights are dependant on climatic conditions (Duviard, 1973) and bugs in the migratory state will fly in the correct conditions of sunshine, sir temperature and atmospheric humidity. Annual observed variations in this migratory phenology led to a hypothesis of a link between migrations and movements of the Inter-Tropical Convergence Zone (I.T.C.Z.). Such an hypothesis was fi~st,but padally, proposed by Golding (1928) and Bowden (1973) later developed this idea with the aid of some results from the work reported here. In the present paper too, the argument will be essentially phenological, strengthened by direct evidence of seasonal displacements, distributions and incidence of colour forms. Review of available evidence The precariousness of habit ats Southwood (1962) discussed the importance of temporary habitats in the evolution of migratory behaviour of insects. Although useful and fitting the case of cotton stainers, this idea is not wholly acceptable in the Tropics (Bowden, 1964). The temporary nature of the habitats of cotton stainers is indicated in the early works on Dysdercus spp., summarised by Pearson (1958) and its importance is assumed in the recent work , of Dingle & Arora (1973) and Dingle (1974), although no field experiments were presented by these authors. - IA B 'DlSTRlBUTlON OF MALVALES IN SAVANNA ISTRlBUTlON OF MALVALES IN RAIN FOREST Adansonlo diglloto - Bomhox buonopozense, Ceiba pentandro. Bombax costatum ._. .Slerculio tropacanlho Sterculia setigero _.._ Fig. I.-Distnbution of the main malvaceous host-plants of Dysdercus spp. in the savanna (A) and forest (B) areas. Within limits of species, lightly dotted areas represent presence of plants and heavily-dotted areas show their optimal range. MIGRATIONS OE DYSDERCUS IN WEST AFRICA 187 Two types of factor are responsible for the precariousness of the habitats of Dysdercus spp. : Food factors.-The host plants (either malvaceous hosts or occasional graminaceous hosts in the case of D. voelkeri Schmidt) provide fruits @bolls) and seeds that are necessary for the development .of Dysderciis (Pearson, 1958). These sources of food are available during a part of the year only. Each host species releases its seeds during a shont period of time (a few weeks to a few months) in a given area and the different plant hosts do not present a similar seeding phenology although, for a given host plant, sharp variations may be observed with changes of latitude. Although the seeding periods of the different host plants are not wholly synchronous, it does noi seem that, when added to each other, they would facilitate the survival of the insects throughout the year at one locality (Whitfield, 1933; Duviard, 1977). Host plants are scattered in the whole non-steppe zone of West Africa (for the names of vegetation zones, see Keay, 1959), ranging from dry Sudan savanna to evergreen rain forest (Fig. 1). Climatic factors.-The sensitivity Of Dysdercns spp. to tempenature and humidity affecting the survival of eggs 'and young nymphs (Pearson, 1958) as well as the activity of nymphs and adult stages (Youdeowei, 1967; Duviard, 1973), is well known. We have previously shown that, in the case of D. voelkeri, favourable climatic conditions are generally encountered together with available food (Duviard, 1973), but this is not a perfect synchrony. During the dry season, the southward movement of the I.T.C.Z. may, with the establishment of the harmattan (a dry warm wind from the Sahara), restrict development of cotton stainers for some time in the savanna habitets, although food may (beavailable. Dryness and high temperature associated with this wind first stop the activiiy of adult and immature stages and eventually kill the eggs and nymphs (Duviard, 1973). In the min forest $belt,areas with much available food will {become unsuitable when rain is heavy. High mortality then occurs due to drowning of colonies of nymphs by rainfall (Galichet, 1956) and to a lethal dungal disease associated with high humidity (Duviard, 1977). Thus, the habitats of Dysdercus (i.e., fruiting host plants and the area of ground where seeds are dispersed), either do or do not exist. If they exist, they may be colonised or not by the insects, depending on climatic conditions. Sudden immigrations Authors working on D. voelkeri (the species referred to '51s D. sziperstitiuszis (F.) until the revision by Pierrard, 1967) in West or Central Africa have observed that, in a given area where the species was completely absent for several months, the pest appeared suddenly in large numbers. It is significant that the term 'immigration', used by Golding (192S), has since been used often in this context. Examination of data reveals two distinct types of immigration : Immigration occurring with the omet of the dry semon.-This type has been more often described fur it is observed in the cotton-growing area during the period when cotton forms bolls land seeds which may be colonised by stainers (Golding, 1928, at Ibadan, Nigeria; Pierrard, 1972, at Bambari, Central African Republic; Gibbs & Leston, 1970, at Tafo, Ghana; Bowden, 1973, at Kwadaso, Ghana; Duviard, 1973, at Foro-foro, Ivory Coast). This immigration follows 'il total disappearance of the species during ithe preceding rainy season, and is observed only in the southern part of the geographicaI area of D. voelkeri. Immigration occurring with tlze onset of reins.-This has seldom been described, for it happens during the absence of cotton crops from the fields (Whitfield, 1933, in the Sudan; Golding, 1928, at Kano, Nigeria: Duviard, this paper, at Ferkessedougou, Ivory Coast; (Issa Kone, personal communication, at Bamako, Mali). It follows a complete disappearance of rhe species during the preceding dry season and is observed only in the northern part of the geographicalarea of D. voelkeri. 188 L70kllhlQUE DUVIAF.1) Thus, in the whole area exploited by D. i90elXeri in West Africa, which is limited in the north by the SOO mm isohyet (according to Whitfield, 1933), there appear to be two distinct areas : a southern one, roughly Guinean and sub-Sudanian, where insects immigrate suddenly with the dry season and disappear with the rains and a northem one, roughly Sudanian, where insects immigrate with the rains and disappear during the dry season.